A computer network is a collection of interconnected computing devices that can exchange data and share resources. Example network devices include layer two devices that operate within the second layer (L2) of the Open Systems Interconnection (OSI) reference model, i.e., the data link layer, and layer three devices that operate within the third layer (L3) of the OSI reference model, i.e., the network layer. Network devices within computer networks often include a control unit that provides control plane functionality for the network device and forwarding components for routing or switching data units.
An Ethernet Virtual Private Network (EVPN) may be used to extend two or more remote layer two (L2) customer networks through an intermediate layer three (L3) network (usually referred to as a provider network), in a transparent manner, i.e., as if the intermediate L3 network does not exist. In particular, the EVPN transports L2 communications, such as Ethernet packets or “frames,” between customer networks via the intermediate network. In a typical configuration, provider edge (PE) network devices (e.g., routers and/or switches) coupled to the customer edge (CE) network devices of the customer networks define label switched paths (LSPs) within the provider network to carry encapsulated L2 communications as if these customer networks were directly attached to the same local area network (LAN). In some configurations, the PE network devices may also be connected by an IP infrastructure in which case IP/GRE tunneling or other IP tunneling can be used between the network devices.
In some examples, EVPNs may include one or more Ethernet Segments. An Ethernet Segment generally may include one or more EVPN instances (EVIs), that each represents a different virtual layer two network with separate forwarding domains. In some examples, an Ethernet Segment may include a CE network device that is multi-homed to multiple PE network devices, such that network links between the PE network device and the CE network device may operate as a single logical network link for transmitting network traffic. In EVPNs, a PE network device typically uses the Border Gateway Protocol (BGP) (i.e., an L3 routing protocol) to perform control plane messaging. For example, a PE network device may announce its presence within an Ethernet Segment by sending an Ethernet Auto-Discovery route using BGP to a multi-homed CE network device. In other examples, a PE network device may advertise to other provider edge network devices MAC addresses learned from local consumer edge network devices to which the PE network device is connected.
In some examples, a customer network includes a CE network device that is multi-homed to multiple PE network devices in a service provider network. The customer network may include hosts and other network devices that send traffic through the CE network device to PE network devices in the service provider network. As one example, a network device within the customer network may run one or more link-state protocols such as Intermediate System-to-Intermediate System Protocol (ISIS) and/or Open Shortest Path First (OSPF). The PE network devices may also similarly run such link-state protocols and exchange control plane messages with the network device within the customer network via the CE network device. Control messages may be uni- or multi-casted between the PE routers and the network device within the customer network. Such control plane messages may have time-to-live (TTL) values of 1 because the network device within the customer network assumes that the PE network devices are a single hop away from the network device.
In active-active (or all-active) configuration, the CE network device runs a load-balancing algorithm to evenly distribute the flow of network packets across all available PE network devices in an EVI of an Ethernet Segment. However, the CE network device may not treat network packets differently based on the packet header or packet contents when running the load-balancing algorithm to forward network packets to the Ethernet segment. Consequently, when a network device within the customer network sends a control plane message destined for one of the PE network devices, the CE network device may, due to its load-balancing algorithm, send the control plane message to a PE network device in the Ethernet Segment that is not the intended destination PE network device. The PE network device that receives the control message may decrement the message TTL value to 0, which causes the packet to be dropped before reaching the intended destination PE network device.